This invention relates to a method and apparatus for airfoil electroplating, and an airfoil with enhanced electroplating thickness and uniformity. The method and apparatus have particular application in regulating and controlling the deposited thickness of platinum and other platinum group metals on high span regions of turbine airfoil components during the platinum electroplating process.
Platinum aluminide coatings are applied to turbine components to provide environmental protection of the nickel substrate base metal. The application of platinum aluminide coatings is a three-step process that includes electroplating, diffusion heat treatment and aluminiding. During electroplating, platinum is plated over the surface of the component to be coated. Diffusion heat treatment creates a metallurgical bond between the nickel substrate and the layer of platinum. Aluminiding is conducted in a furnace at elevated temperatures where the platinum on the surface of the part is reacted with an aluminum vapor that creates a platinum aluminide coating.
A design challenge that is optimized during the development of a platinum aluminide coating process for a part is to minimize the thickness variation of the coating on the part. The variation in platinum aluminide coating thickness is a function of a platinum thickness and aluminum activity in vapor phase aluminide (VPA) retort. As platinum thickness increases, platinum aluminide thickness increases. As aluminum activity increases, platinum aluminide coating thickness increases. During platinum plating, parts are immersed within the plating tank with the bottom of the part attached to the cathode fixture and the top of the part submerged deepest in the tank. For a turbine blade, the bottom of the blade is the dovetail, which is not exposed to plating electrolyte while the tip of the blade is submerged deepest. Independent of electroplating anode design, the surfaces of the part that are deepest in the tank will plate thicker than the parts towards the top of the tank due to decreased temperature and flow rate of electrolyte at the top of the tank. Within the VPA retort, the aluminum vapor along the height of the part has a gradient of activity, low activity towards the bottom and higher activity towards the top. The combined effects of the platinum thickness variation in the plating tank and aluminum activity in the VPA retort have historically made uniform coating thickness distributions hard to achieve.